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Transcript
+ Describing Motion Position (x), Velocity (v), and Acceleration (a) An object continues in its state of motion until acted on by an outside force (Newton’s First Law). Acceleration is equal to the outside force applied divided by the inertia of the object (Newton’s Second Law). An object’s change in momentum equals the outside force multiplied by the duration of the force (Impulse). An isolated system’s momentum does not change, so momentum can be transferred or shared between objects (conservation of momentum). + Time for Work Work in Physics https://youtu.be/RSssClMtrx0 + What Is “Work” In Physics? Work is done when a net force acts on an object over some distance. Work is the product of the force on an object and the distance through which the object is moved. + Impulse vs. Work Impulse Impulse describes a force applied over some time. Work Work describes a force applied over some distance. + Doing Work + Working hard or hardly working? Work is done lifting the barbell. If the barbell was lifted twice as high, it would take twice as much work. How much work does the body builder do when he holds the barbell above his head? + Working hard or hardly working? Work is done lifting the barbell. If the barbell was lifted twice as high, it would take twice as much work. Work is not done if the barbell does not move. + Team Questions: Is work done on the object in the following situations? 1. A teacher applies a force to a wall and becomes exhausted. 2. A book falls off a table and free falls to the ground. 3. A rocket accelerates through space. + Units for Work The unit for work is the Newton-meter (N•m), also called the joule (J). One joule (J) of work is done when a force of 1 Newton is exerted over a distance of 1 m (e.g., lifting an apple over your head). Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package? Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package? What would happen to the amount of work if: (a) the distance applied doubled? (b) the magnitude of the force tripled? + Work Changes An Object’s “Energy” + Work Changes An Object’s “Energy” When work is done by an archer in drawing back a bowstring, the bent bow acquires the ability to do work on the arrow. + Work Changes An Object’s “Energy” When work is done by an archer in drawing back a bowstring, the bent bow acquires the ability to do work on the arrow. When work is done to raise the heavy ram of a pile driver, the ram acquires the ability to do work on the object it hits when it falls. + Work Changes An Object’s “Energy” When work is done by an archer in drawing back a bowstring, the bent bow acquires the ability to do work on the arrow. When work is done to raise the heavy ram of a pile driver, the ram acquires the ability to do work on the object it hits when it falls. When work is done to wind a spring mechanism, the spring acquires the ability to do work on various gears to run a clock, ring a bell, or sound an alarm. + Work Changes An Object’s “Energy” In these examples, something is acquired by the objects that gives them the ability to do more work. The property of an object or system that enables it to do work is called energy. Like work, energy is measured in joules. Energy due to an object’s motion or position is known as mechanical energy. Just like an impulse changes the momentum of a system, doing work on a system changes the system’s energy. + Work is force over a distance and it changes an object’s mechanical energy.